Over the past thirty years, significant attention has been given to the production of ethyl alcohol, or “ethanol,” for use as an alternative fuel. Ethanol not only burns cleaner than fossil fuels, but also can be produced using grains such as corn, which are of course renewable resources. At present, approximately sixty-nine “dry milling” plants in the United States produce over two billion gallons of ethanol per year. Additional plants presently under construction are expected to add over four hundred million gallons to this total in an effort to meet the current high demand.
As noted in the foregoing discussion, a popular method of producing ethanol is known as “dry milling,” and in the United States is typically practiced using corn. As is well known in the industry, the dry milling process utilizes the starch in the corn or other grain to produce the ethanol through fermentation, and creates a waste stream comprised of byproducts termed “whole stillage” (which may be further separated into products known as distillers wet grains and “thin stillage”). Despite containing valuable oil, this whole stillage has for the most part been treated as waste and used primarily to supplement animal feed (mostly in the form of distillers dried grains with solubles (DDGS), which is created by evaporating the thin stillage, recombining the resulting concentrate or syrup with the distillers wet grains, and drying the product to have a low moisture content; see, e.g., U.S. Pat. Nos. 5,662,810 and 5,958,233,the disclosures of which are incorporated herein by reference).
Efforts to recover the valuable oil from this byproduct have not been successful in terms of efficiency or economy. For example, one approach involves attempting to separate the oil from the thin stillage before the evaporation stage, such as using a centrifuge. However, spinning the thin stillage at this stage does not produce usable oil, but rather merely creates an undesirable emulsion phase requiring further processing. Moreover, the volume of thin stillage present is generally 2 to 10 times greater than the syrup, which requires considerable capital to purchase the number of centrifuges required. Together, these obstacles make attempts to recover oil from thin stillage highly inefficient and uneconomical.
U.S. Pat. No. 5,250,182 (the disclosure of which is incorporated herein by reference) describes the use of filters for removing substantially all solids and recovering lactic acid and glycerol from the thin stillage without the need for evaporation. Despite eliminating a step in the conventional process, the proposal results in a more complicated arrangement requiring multiple filtration steps. Wholesale elimination of the evaporator in the vast majority of existing plants is also unlikely and otherwise uneconomical. Filters, and especially the microfiltration and ultrafiltration types proposed in this patent, are also susceptible to frequent plugging and thus disadvantageously increase the operating cost. For these reasons, the filtration process proposed in this patent has not gained widespread commercial acceptance.
Accordingly, a need exists for a more efficient and economical manner of recovering oil from a byproduct containing it, such as thin stillage created during the dry milling process used to produce ethanol.